1. Field of the Invention
The present invention relates to a pulse width modulation (PWM) boost system and a start-up method thereof. More particularly, the present invention relates to a PWM boost system with a function of current limit soft-start and a start-up method thereof.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
FIG. 1 shows a conventional PWM boost system 1, which includes a boost circuit 10, a pulse width modulation circuit (PWM circuit) 11, a pre-oscillator 12, a comparator 13, a voltage dividing circuit 14, and a stabilizing circuit 15. FIGS. 2(a) and 2(b) are diagrams of relevant signals when an output voltage Vout of the PWM boost system 1 is connected to a light load and a heavy load, respectively. Signals Vout, VEO1, and IL1 stand for a DC output voltage of the PWM boost system 1, a voltage on a node EO1 connecting the comparator 13 and the PWM circuit 11 (i.e., an error voltage), and an inductor current flowing through a boost inductor L1 in the boost circuit 10, respectively. The stabilizing circuit 15 includes a resistor R3 and a capacitor C2 strung between the node EO1 and a ground terminal.
Referring to FIG. 2(a), when the PWM boost system 1 is started, a reference voltage Vref is applied on a non-inverting input terminal of the comparator 13, while an inverting input terminal of the comparator 13 is connected to a feedback voltage VFB from the voltage dividing circuit 14, so as to define the magnitude of the DC output voltage Vout. When the DC output voltage Vout is lower than a first predetermined voltage Vuvlo (undervoltage lockout voltage) (i.e., in a pre-oscillation period), the pre-oscillator 12 outputs a pre-oscillation signal SOSC to the PWM circuit 11 to generate a PWM signal SPWM. It should be noted that during pre-oscillation period, the comparator 13 does not output a signal (i.e., the level of VEO1 is 0). The PWM signal SPWM is used to change the turn-on or turn-off time of a switch SW1, such that the inductor current IL1 generated by a first voltage Vin and flowing through the boost inductor L1 charges a capacitor C1 intermittently, and the charges stored in the capacitor C1 can generate the DC output voltage Vout. Here, the diode D1 limits a discharging direction of the capacitor C1. After entering a PWM period, the DC output voltage Vout is maintained at the first predetermined voltage Vuvlo for a period of time. When the DC output voltage Vout increases, an inrush current is generated with the inductor current IL1, and the inductor current IL1 does not decline until the DC output voltage Vout reaches a second predetermined voltage Vref×DIV, where DIV=(R1+R2)/R2.
Referring to FIG. 2(b), the operation of FIG. 2(b) during pre-oscillation period is the same as that of FIG. 2(a). However, after the pre-oscillation period is terminated, as the output of the PWM boost system 1 is connected to a heavy load, the signal VEO1 continues switching between two operating modes of the pre-oscillation period and a PWM period, and thus an appropriate PWM signal SPWM cannot be generated by the PWM circuit 11. As a result, the DC output voltage Vout continues oscillating about the first predetermined voltage Vuvlo, and cannot reach the second predetermined voltage Vref×DIV (i.e., the PWM boost system 1 cannot be started up successfully).